Spin-glass freezing in Kondo-lattice compounds

Abstract

A theory is presented that describes a spin-glass phase at finite temperatures in Kondo-lattice systems with an additional Ruderman–Kittel–Kasuya–Yosida interaction represented by long range, random couplings among localized spins as in the Sherrington–Kirkpatrick (SK) spin-glass model. The problem is studied within the functional integral formalism where the spin operators are represented by bilinear combinations of fermionic (anticommuting) Grassmann variables. The Kondo and spin-glass transitions are both described with the mean-field–like static ansatz that reproduces good results in the two well-known limits. At high temperatures and low values of the Kondo coupling there is a paramagnetic (disordered) phase with vanishing Kondo and spin-glass order parameters. By lowering the temperature, a second order transition line is found at TSG to a spin-glass phase. For larger values of the Kondo coupling there is a second order transition line at roughly Tk to a Kondo ordered state. For T<TSG the transition between the Kondo and spin-glass phases becomes first order.